1. Field of the Invention
The present invention relates to a developing apparatus which performs development using an electrophotographic system and which has a plurality of the developer bearing members, and to an image forming apparatus using the developing apparatus.
2. Description of the Related Art
As an image forming apparatus such as an electrophotographic copying machine, a developing device using a magnetic brush method of two-component developing method is known. Also, a method of development in which one developing sleeve (developer bearing member) is used for one photosensitive drum (image bearing member) is generalized.
However, in the recent demand of high-speed to a copying machine, when the speed of the rotational movement of the photosensitive drum becomes faster, a preferable image is not necessarily formed with only one developing sleeve.
Thus, there is a developing device in which a plurality of developing sleeves are disposed to be adjacent to each other such that peripheral surfaces of the developing sleeves are close to each other. With this configuration, it is possible to enhance the capability of development by the prolonged developing time because the developer is conveyed continuously on the peripheral surfaces of the developing sleeves. This system is referred to as a multi-stage magnetic brush development system.
Next, a conventional developing device of a multi-stage magnetic brush development system which includes two developing sleeves will be explained. FIG. 10 is an explanatory view of a developing device of a conventional multi-stage magnetic brush development system.
The developing device 104 includes a developing container 122. The developing container 122 is partitioned into the developing chamber R1 and the stirring chamber R2 by the partition 123.
The conveying screw 124 is disposed in the developing chamber R1 for stirring and conveying the developer. The conveying screw 125 is disposed in the stirring chamber R2 for stirring and conveying the developer in the opposite direction to the conveying screw 124. The developer is circulated and conveyed while being passed at both ends of the developing chamber R1 and the stirring chamber R2.
At the opposing portion of the photosensitive drum 101 in the developing container 122, two developer bearing members of the first developing sleeve 126 and the second developing sleeve 128 are provided. The first magnet roller 127 is fixedly disposed in the first developing sleeve 126 and the second magnet roller 129 is fixedly disposed in the second development sleeve 128. The layer thickness regulating blade 121 is disposed oppositely to the first developing sleeve 126 in the upstream side of the portion facing the photosensitive drum 101 on the first developing sleeve 126.
With this configuration, the layer thickness of the developer supplied from the conveying screw 124 in the developing chamber R1 to the first developing sleeve 126 is regulated by layer thickness regulating blade 121. Then, the developer is used for the first development at the opposing portion between the photosensitive drum 101 and the first developing sleeve 126. Then, the developer is passed to the second developing sleeve 128 and is further used for the second development. The developer used for the second development at the second development sleeve 128 is returned to the stirring chamber R2.
Thus, in the multi-stage magnetic brush development system, it is possible to obtain high development efficiency by performing development twice.
However, at the region where the first magnetic roller 127 and the second magnet roller 129 are opposed in the developing device having two developing sleeves as described above, the magnetic flux density between them becomes high and the developer restraining force by magnetic force increases. Thus, with the interaction of the magnetic force by the two magnet rollers and the driving force of the rotation of the developing sleeves, the space at the developing container side between the first developing sleeve 126 and the second development sleeve 128 is likely to be filled with the developer.
The developer filled in this space is the one which remains after developing is completed. Thus, the developer is in a state where the toner density is lower than the developer immediately after the developer has passed through the layer thickness regulating blade 121. The developer filled in the space is conveyed back to the second developing sleeve 128 by the rotation of the second developing sleeve 128 and effect of the magnetic force of the second magnet roller 129. Then, the developer coating amount of the second developing sleeve 128 becomes much larger than that of the first developing sleeve 126 regulated by the layer thickness regulating blade 121.
As explained above, when the developing process of an electrostatic latent image on the photosensitive drum 101 by the second developing sleeve 128 is performed in the state in which the developer of which toner density is lowered is mixed, the formation of image failure such as density reduction may occur. In addition, when the coating amount of the second developing sleeve 128 is greater than the appropriate value, an overflow of the developer from the developing container 122 occurs and there is a risk of splashing of the developer inside the apparatus.
As a countermeasure of this problem, U.S. Patent Application Publication No. 2004/136755 A1 discloses the provision of the regulating member for regulating the entry of the developer into the space between the first development sleeve 126 and the second development sleeve 128. As shown in FIG. 10, the regulating member 130 is provided in the region between the first developing sleeve 126 and the second developing sleeve 128 for preventing drag motion of the developer. As a result, the developer after completion of the development is prevented from entering the space from the second developing sleeve 128.
However, in FIG. 10, when the regulating member 130 is provided, new problems occur as follows.
The regulating member 130 mechanically blocks out the entering of the developer after development between two developing sleeves. The developer after development is subjected to thrust force by the magnetic force of the magnet roller and the rotation of the developing sleeve. Therefore, for peeling off the developer on the second developing sleeve 128, the mechanical pressure which is larger than the combination of the magnetic force and the thrust force must be applied. Then, the developer near the regulating member 130 can be hardened by the pressure of the regulating member 130 and the developer may aggregate.
Agglomerates fall in the stirring chamber R2 from the vicinity of the regulating member 130 and are conveyed from the stirring chamber R2 to the developing chamber R1 by the conveying screw 125. Thus, the agglomerates of the developer are borne on the first developing sleeve 126 and thereafter are finally caught by the layer thickness regulating blade 121. Thereby, the appropriate coating for the first developing sleeve 126 of the developer is affected and a so-called white stripe-like image may occur.